Sunday, 15 December 2013

Q&A 2013

These are not koalas

This time last year, I took a number of questions typed into Google that had brought people to this blog, and did my best to answer them. Obviously, it's not helping the people that actually asked the questions, most of whom had done so months before, but they may not be alone, and besides, it's a fun change of pace. So I'm going to do it again this year, before taking my customary end-of-year break.

A lot of the questions that showed up in the blogger interface were the same this year as last. With "is a rabbit a rodent?" topping the poll, as ever. I'll skip on the repetition, though, and look at some of the new ones. Whether or not that will leave enough new questions for next year is anyone's guess, but at least there's enough for now. So here goes.

How are Marsupials and Dinosaurs related?

This might sound a bit odd, but all animals are related if only you go back far enough, so it's a perfectly reasonable question. But just how far back do you have to go, and are there are any other interesting steps in between? For instance, a similar question that showed up was "Are giant otters and seals related?" Well, yes, but then humans are, if you go back a sufficient distance, related to artichokes.

In fact, though, giant otters, like all otters, belong to the weasel family, while seals have a family of their own. So, badgers, say, are closer to giant otters than seals are, which is probably the sort of thing the questioner was actually wanting to know - there are things that aren't at all seal-like that separate the two, so they're not really close relatives. Having said which, the weasel and seal families do both belong to a broader group called the 'dog-like carnivores', so there is at least something of a similarity.

But marsupials and dinosaurs? How far back do we have to go to find the link there? For a start, we have to understand that marsupials are mammals, and mammals are a real biological group. That is, all mammals (including marsupials) are more closely related to all other mammals than they are to anything that's not a mammal. Like, say, a dinosaur. So, the question might as well be "how are mammals and dinosaurs related?" The answer is going to be exactly the same. For that matter, since birds are dinosaurs (yes, really), it's also the same question as "how are mammals and birds related?"

The answer is 'not very closely'. We have to go back a mind-bogglingly long way to find anything that's a common ancestor of both marsupials and dinosaurs (or birds and mammals). Almost as far, in fact, as the very first vertebrate that did away with tadpoles and so severed its connections to the water. Much, much, further back than the first dinosaurs. The split between the Synapsida (mammals) and Diapsida (birds and most, or maybe all, living reptiles) is just about the oldest one we know of within these truly land-dwelling vertebrates.

In short, while marsupials and dinosaurs are, like everything else, related, they're hardly what you'd call close. Examples of things that are closer to the likes of T rex than to marsupials include crocodiles, snakes, and, yes, pigeons.

Will Marsupials Go Extinct?

Technically, the answer is 'yes', because, so far as we know, so will anything if you wait long enough. But 'long enough' in this context could be millions of years. What about a shorter term than that?

There are approximately 300 species of marsupial. According to the International Union for the Conservation of Nature, whose job it is to know such things, around one in five of them are currently threatened with extinction. Of those, 42 are officially listed as 'endangered species', including about fifteen that are right on the edge of extinction. So, in one sense, the answer is "yes, in all likelihood, some of them will." Which is pretty depressing. On the other hand, over half of marsupial species appear to be safe for the time being, so it's not as if the entire group is going to disappear any time soon.

But I can't help suspecting that that isn't what the questioner meant, and that also relates back to the first question I answered. Because I doubt that that questioner was thinking of the likes of house sparrows when he asked about dinosaurs.

I suspect this is really to do with the general perception that marsupials are "more primitive" than placental mammals, and will inevitably be driven to extinction if the two ever meet for a protracted period of time. But is this true, and what do we mean by 'primitive' anyway?

We can't easily tell whether or not an animal had a pouch by looking only at its fossils. Fortunately, there are a number of skeletal differences between marsupials and placental mammals, and we can at least look at those. Doing so, we find that the first marsupial-like animals were, indeed, very ancient, and lived alongside some of the earlier dinosaurs. Thing is... so did the first placental-like mammals.

Indeed, marsupials and placental mammals are two different branches of the same tree. By definition, they've been in existence equally long, and they've both been evolving during the intervening period. The first marsupials wouldn't have looked much like a kangaroo, for instance. In fact, a kangaroo is a pretty specialised animal in some respects, with a number of complex adaptations. So, really, it's no more 'primitive' than a deer or a baboon.

Having said that, it does seem likely that marsupial reproduction has changed less since they first appeared than has our own, and that does appear to be something of a disadvantage for them. Until humans and dingos arrived, the only major group of placental mammals on Australia were bats. Since marsupials can't fly, they weren't really competing, and everyone was happy. Australia is full of marsupials because there weren't any placentals there to oust them. Right?

Well, partly. Because there are marsupials in the Americas, including the Virginia opossum (or simple "'possum") in parts of the US. And there aren't any in Europe or Africa because there never have been - they didn't hang around long in Asia, either. Certainly, placentals have out-competed marsupials in the Americas, and there really aren't that many of the latter. But that there's any at all means that they won't necessarily, always beat them.

Marsupials are not some evolutionary dead-end, and they're pretty good at what they do.

Hoofed Animal Questions

Are Pigs Hooves Cloven?
Yes, they are. Because they do have cloven hooves, but, unlike almost every other cloven-hoofed animal, don't chew the cud, pig-meat is not considered kosher. By the same reasoning, neither is the meat of peccaries, the wild American equivalent of pigs, which also have cloven hooves.

What's the Biggest Cloven-Hoofed Animal?
The giraffe. Although there are some pretty big wild cattle, too, such as bison.

Do Horses Have Four Stomachs?
No, but see here for what they have instead. Which is, essentially, not too far off from a second stomach.

What's the Closest Relative of the Musk Ox?Musk oxen are members of the cattle family, which probably isn't a surprise. But, within that, they belong to the goat-like animals, not the cow-like ones, which may be less obvious. There is some dispute as to what exactly their closest living relatives might be, but a best guess seems to be the serows and gorals of eastern Asia.

How Long was the Age of Mammals?

The 'Age of Mammals', more properly called the Cenozoic, is the term for that period of Earth's history since the extinction of the non-avian dinosaurs. It's the time when mammals, rather than reptiles, have become the most obvious large, ground-dwelling animals on Earth. There are actually more living bird species than mammalian ones, and a similarly huge number of small lizards and the like. Not to mention fish in the seas, and whoppingly vast numbers of small creepy-crawlies. But it's the mammals we tend to notice, and, while they certainly existed long, long, before Triceratops and its kin bit the dust, they had to wait for an asteroid strike before they could inherit the Earth.

A lot of sources will give the answer to the question as '65 million years', but the latest estimates put it back a little bit further, to 66 million years. And counting.

What's the Strangest Organism?

Obviously, this depends a lot on what you happen to find strange! There's some really freaky invertebrates out there, and some fairly bizarre fish, too. A recent study has shown that the animal group least related to any other, representing the earliest branch in the entire family tree of the animal kingdom that we know of, is that of the comb jellies. (We'd previously thought it was sponges). So those are pretty odd. But, since this is Synapsida, let's stick to mammals.

Personally, I think naked mole rats are pretty odd. When you think about it, so are elephants. I mean, an animal that picks up objects with its nose? Come on! Manatees are strange, and so are pangolins, which look like pine cones with legs. The xenarthrans - armadillos, sloths, and anteaters - are also unusual, and have some quirky internal anatomy to boot. Among placental mammal species, the aardvark seems to be the one with the smallest number of close living relatives, and, let's face it, it's kind of peculiar-looking, too. Among marsupials, the marsupial mole is also fairly strange.

But, by gosh, you've got to go some way to beat the platypus...Synapsida will return in the New Year.

5 comments:

Re: "there aren't [marsupials] any in Europe or Africa because there never have been"Well, not recently, but I think a few marsupial fossils from the early Cenozoic (Eocene?) are known from Europe and North Africa.(Sorry to be a nit-picker:I really like "Synapsida"!)

Bats were hardly the only placental mammals in Australia before the arrival of humans. There actually is a murid radiation in Australia. These rodents are likely to have reached the continent in Miocene times. Darren Naish once blogged on them.

As Allen Hazen points out, marsupials (or better, metatherians) were present in the Paleogene of Europe, North America and Africa at least. In fact, the North American ones only appear to have died out in the Miocene.

As you rightly point out, it's difficult to tell from fossil mammals if they had pouches or not. Given that many traditional placentals from the Paleogene have been argued to be outside Eutheria proper and we do not know if placentals evolved from pouchbearing mammals, there is the distinct possibility that at least some of these stem-eutherians had pouches. Of course, we're talking about 'marsupial' in terms of having a pouch here, not in terms of phylogeny.